Aluminum coated steel

ABSTRACT

An aluminum coated low alloy mild steel sheet material and a method of making said material which contains a small amount of titanium sufficient to precipitate all the carbon in the low carbon steel and provide a slight excess of uncombined titanium dissolved in the steel and containing molybdenum in an amount less than 1 percent by weight with the balance being mainly iron and incidental impurities. The aluminum coated material exhibits extraordinarily high creep resistance and rupture strength for a low carbon steel material at a service temperature above 1,250*F (i.e. 1,300*F) in an oxidizing atmosphere.

United States Patent [1 1 Hughes et al.

[451 May 6,1975

[ ALUMINUM COATED STEEL [73] Assignee: Inland Steel Company, Chicago,111.

22 Filed: Apr. 19, 1913 21 Appl. No.: 352,597

[52] US. Cl 29/1962; 117/71 M; 117/114 C [51] Int. Cl..... B32b 15/18;B32b 15/20; C230 1/08 [58] Field of Search 29/1962; 117/71 M, 114 CFOREIGN PATENTS OR APPLICATIONS 1,391,659 11/1965 France 75/124 PrimaryExaminer-L. Dewayne Rutledge Assistant Examiner-Arthur J. SteinerAttorney, Agent, or Firm-Hibben, Noyes & Bicknell 57 ABSTRACT Analuminum coated low alloy mild steel sheet material and a method ofmaking said material which contains a small amount of titaniumsufficient to precipitate all the carbon in the low carbon steel andprovide a slight excess of uncombined titanium dissolved in the steeland containing molybdenum in an amount less than 1 percent by weightwith the balance being mainly iron and incidental impurities. Thealuminum coated material exhibits extraordinarily high creep resistanceand rupture strength for a low carbon steel material at a servicetemperature above 1,250 F (i.e. 1,300 P) in an oxidizing atmosphere.

11 Claims, 1 Drawing Figure PATENTEDH Y E WEIGHT GAIN, rrz /cm u CYCLICOXIDATION TO |5oo l l l l l J 2 3 4 5 e 7 VTIME, DAYS ALUMINUM COATEDSTEEL The present invention relates generally to a steel articlesuitable for service at an elevated temperature above 1,250F. and moreparticularly to an aluminum coated low alloy mild steel sheet which hasgood resistance to oxidation at temperature of l,500F and which also hasunusually high creep resistance and rupture strength at temperatures ofl,300F while at the same time having good formability at roomtemperature.

It is important to provide a steel sheet or the like which can be usedat an elevated temperature of about l,500F in an oxidizing atmospherefor extended periods without undergoing destructive oxidation andwithout resorting to the use of high alloy steels. One method ofincreasing the oxidation resistance of steel has been to provide analuminum coating on the steel sheet. However. when an aluminum coatingcontaining silicon (Type 1 aluminum) is applied to mild or low carbonsteel and heated. excessive subsurface oxidation of the steel occurs attemperatures above l,300F. Such aluminum coated steels are unsuited forservice of even a few days at temperatures above 1,300F. Also, a typicalaluminum coated mild steel such as AlSl L008 steel having asubstantially pure aluminum coating (Type [I aluminum) is notrecommended for extended use at temperatures above about l,lF.

A further very important requirement of a steel sheet or like articleintended for service at elevated temperatures is that the article havegood mechanical properties at the elevated service temperature (i.e.must be resistant to creep and rupture) and at the same time the articleshould have good formability at room temperature. The high temperaturestrength required is important to insure structural integrity of thearticle after prolonged service at the elevated temperature, and theroom temperature formability is important in order to facilitatefabrication.

Heretofore, stainless steel alloys containing at least 12 percentchromium have generally been used where the extended service life is tobe in an oxidizing atmosphere at temperatures of about l.500F and wheregood high temperature physical properties are required. Stainless steelalloys, however. are relatively expensive to produce and are often tooexpensive for many applications.

It is therefore an object of the present invention to provide analuminum coated low alloy steel article which is suitable for long termservice in an oxidizing atmosphere at elevated temperatures and which isless expensive to produce than stainless steel.

It is a further object of the present invention to provide an aluminumcoated low carbon steel sheet having good resistance to oxidation atl.500F and high creep resistance at elevated temperatures without usinga high percentage of alloying elements in the steel.

It is another object of the present invention to provide an aluminumcoated low alloy mild steel sheet having resistance to oxidation atl,500F and good rupture strength at elevated temperatures which alsoexhibits good room temperature formability.

Other objects of the present invention will be apparent to those skilledin the art from the detailed description and claim to follow when readin conjunction with the accompanying drawing which shows the weight gainofa mild steel panel embodying the present inven tion and a stainlesssteel panel over a prolonged period of cyclic oxidation at an elevatedtemperature.

it has been found that a steel article having good mechanical propertiesand high resistance to surface and subsurface oxidation when heated atan elevated temperature in an oxidizing atmosphere can be providedeconomically without using large amounts of expensive alloying elementsin the steel by applying a thin aluminum coating to a low carbon steel(0.03 to 0.10 percent by wt. C) containing both a small amount ofmolybdenum (i.e. less than 1 percent by wt.) and an amount of titaniumwhich is sufficient to precipitate all of the carbon and nitrogencontained in the steel and leaving a small excess of uncombined metallictitanium in solution in the steel. Preferably the excess of uncombinedtitanium remaining in the steel is an amount between about 0.1 and 0.3percent by weight of the steel. Since the weight percent of titaniummust be approximately four times the weight percent of carbon in thesteel tie. the amount of nitrogen present being relatively small) inorder to precipitate essentially all the carbon in the steel, thetitanium content of the substrate steel sheet in the present inventionpreferably is at least four times the carbon content of the steel plusan additional amount of titanium sufficient to provide about 0.l to 0.3percent by weight uncombined titanium. The titanium content can be asmuch as ten times the weight percent of carbon in the steel. but anamount of titanium greater than that specified herein gives no increasedbenefits and merely adds unnecessarily to the cost. Since the totalamount of carbon and nitrogen in a low carbon steel of the typeconventionally used for producing aluminum coated steel sheet materialis small, generally less than 0.1 percent, the total amount of titaniumrequired in the present invention is also small, comprises less than Ipercent by weight of the steel, and will generally fall between about0.35 percent and 0.70 percent by wt. of the steel.

[t has been found that when a small amount of molybdenum is alsocombined with the low carbon steel containing sufficient titanium tocombine with all of the carbon and nitrogen in the steel and leaving asmall excess of uncombined titanium in solution in the steel. theresulting low alloy steel exhibits good formability properties at roomtemperature and has extraordinarily good high temperature strengthproperties, including good resistance to creep and high rupturestrength. These high temperature strength properties are very unusual ina low carbon steel. The amount of molybdenum used in the titanium alloysteel which is devoid of free carbon preferably ranges between about 0.1and 0.70 percent by weight of the steel.

A preferred method of aluminum coating a low carbon steel strip havingthe molybdenum and titanium content thereof in accordance with thepresent invention is by a hot-dip coating process generally known in theart as a Sendzimir-type process. wherein a continuous steel sheet orstrip which is free of scale and rust is fed continuously from a coilthrough a furnace containing an oxidizing atmosphere maintained at atemperature between about 330F and 930F which burns off any oil residueon the surface of the strip and forms a thin surface oxide film. Theoxide coated steel sheet then passes through a furnace containing areducing atmosphere. such as the hydrogen-containing HNX atmosphere,having a temperature between about l,500F and l.800F and preferablybetween l.600F

- l,750F; whereby the oxide coating on the strip is reduced to form asurface layer of metal free of nonmetallic impurities to which moltenaluminum readily adheres. Following the reducing step, the strip is fedinto a hot-dip aluminum coating bath through a protective hood whichprevents the reduced metal surface being oxidized before entering thecoating bath. The aluminum coating bath, for example, can besubstantially pure aluminum (Type ll aluminum coating) or an aluminumrich alloy, such as aluminum containing up to l l percent by wt. silicon(Type l aluminum coating). After leaving the hot-dip aluminum coatingbath, the coating thickness on the strip is regulated by a pair ofoppositely disposed thickness-regulating jet wipers or rolls whichproduce a uniform thin aluminum coating. and the strip is cooled by anysuitable means. The aluminum coated strip is then wound into a coil.Conventional Sendzimir-type process apparatus can be used in each of theprocessing steps.

The step of burning off the oil and oxidizable combustible material onthe surface of the steep strip before the strip is subjected to thereducing atmosphere can be omitted, if desired, provided the strip isotherwise thoroughly cleaned, immediately prior to the reducing step, asby conventional alkaline cleaning and pickling.

The two graphs in the accompanying drawing show the total weight gainper square centimeter of exposed area due to surface and subsurfaceoxidation of a (A) hot dip aluminum content steel sheet embodying thepresent invention and containing 0.05 percent by weight carbon, 0.50percent molybdenum, and 0.35 percent by weight titanium having a Type Ialuminum coating ofa thickness of 2 mils and (B) a panel of stainlesssteel 409 (SAE 5 I409), both heated in air at l,500F over the indicatedperiod of time. The aluminum coated molybdenum-titanium containing lowalloy steel sheet (Curve A) exhibits no weight loss due to spalling,retained its integrity and continue to show a uniform oxidation ratethroughout the total length of l0 atmosphere, since there is nocontinuous subsurface barrier layer of oxide formed on prolonged heatingin an oxidizing atmosphere, as in the usual aluminum coated low carbonsteel sheet, and only discreet particles of metallic oxide are formedbelow the surface of IS the steel in the form of a discontinuoussubsurface stratum which does not impede the uniform diffusion of thealuminum into the steel surface.

A low alloy mild (ie low carbon) steel suitable for hot or cold rollinginto sheets and aluminum hot-dip 20 coating in accordance with thepresent invention falls within the following approximate composition:

Percent by Weight C 0.035 0.10 N .(llO

25 My 0.30 0.70 Ti 0,30 0.70 Mo 0. l0 0.70 Al 0.03 0.09

Fe and incidental impurities Balance In order to further characterizethe article of the present invention panels of steel having thecomposition indicated in the following Tables were tested to determinetheir mechanical properties at room temperature and at a temperature ofl,300F. in Tables l and II each of the test panels of low alloy aluminumkilled low carbon steel having the indicated chemical analysis wassubjected to a preheating cycle identical to that which a steel sheetwould be subjected to in the course of continuous hot-dip aluminumcoating in a Sendzimir-type coating process, and thereafter the panelswere subjected to standard tensile strength tests with the followingresults:

TABLE l Properties of Experimental Alloys Preheat LYS or Alloy SteelsComposition Temp. 0.2 PS UTS 0.2 PS E,- E

C Ti Mn Al Mo (F) (ksi) (ksi) UTS ('1) (1) Ti-l 0.045 0.34 0.32 0.036I600 Zll SI .6 0.43 2| .3 35.l i750 21.8 48.0 0.45 20.3 35.6 Ti-lMo(0.50) 0.06 0.35 0.35 0.036 .50 1600 26.1 54.5 0.48 l8.3 29.8 H 220518 0.42 2L2 33.9 MF-l (SAE 5l409) 40.0 68.0 0.66 30.0

Containing in addition the normal amounts of impurities: N. S. P. Si

TABLE ll Tensile Properties of Experimental Alloys at 1300F PreheatAlloy Composition Temp. 2 PS UTS 0.2 PS E,-

C Mn Ti Al Mo ("Fl (ksi) (ksi) UTS (il l Ti-l 0.045 0.34 0.32 0.03616-00 7.8 9.5 0.82 77.5 l750 9.8 l 1.2 0.88 530 TH M0 006 0.35 0.350.036 0.50 I600 I05 I38 076 42.0 I750 l 1.5 I48 078 38.0 MF-l (SAE51409) ll.0 12.0 0.92 36.0

Containing in addition the normal amounts of impurities. N. S. P. Si

It will be evident from the foregoing data that the low alloy mild steelbase of the present invention containing 0.35 percent by wt. titaniumand 0.50 percent by wt. molybdenum exhibits no significant loss of roomtemperature ductility as compared with a like low alloy steel withoutmolybdenum while exhibiting substantially improved high temperaturetensile strength. The tensile strength properties of the low alloytitaniummolybdenum low carbon steel compares favorably with the mufflergrade stainless steel (SAE 5 I409) used as a reference standard. Byadjustment of the carbon. titanium and molybdenum content within theherein disclosed ranges, it is possible to provide a steel which hasgood high tensile strength while retaining adequate room temperatureductility for forming purposes and which has the further advantage ofbeing produced by a standard hot-dip aluminum coating procedure, such ason a conventional Sendzimir continuous hot-dip coating line.

Test hot-dip aluminum coated low alloy mild steel panels embodying thepresent invention and having the composition indicated in Table I weresubjected to standard stress rupture and creep strength tests at l.300Fin air. and the results obtained are shown in the following Tables Illand IV:

TABLE ll] by the greatly reduced creep rate at the selected stresslevels at l,300F. These improvements in the high temperature strengthproperties are extraordinary in a low carbon steel and are veryimportant in expanding the utility and applications of the low carbonsteels.

We claim:

I. An aluminum coated low alloy mild steel article adapted for serviceat an elevated temperature comprising. a low-alloy aluminum killed lowcarbon steel sheet which contains uniformly distributed throughout as anessential combination of alloy elements titanium in an amount up to amaximum of 1% by wt. which combines with all the carbon and nitrogen andleaving uncombined titanium throughout the steel and molybdenum in anamount between about 0. l0 percent and 0.70 percent by wt. of saidsteel, a uniform thin coating of metallic aluminum directly on a surfaceof said steel sheet which is free of surface oxides and non-metallicimpurities. said article in the as coated condition exhibiting goodformability at room temperature and high creep resistance and rupturestrength at an elevated temperature of l300F l500F. and said articleafter heating in an oxidizing atmosphere at a temperature of 1500F andsaid article having aluminum uniformly dif- Creep Data for ALUMINIZEDTITANIUM AND TlTANlUM-MOLYBDENUM ALLOY STEELS at l300F IN AIR SteelStress Level Time to Creep hrs.) Minimum Creep Rate Elongation atRupture (psi) 0.5; 1.0] 5% ('7? Hr.) '1

Titanium 3000 30 65 375 .0l3 l7 (Ti-l 4000 10 I04 .044 28 5000 2.5 4.0I86 .26 65 Titanium 4000 5 40 740 .0048

Molybdenum 4500 I5 310 .012 28 (TH Mo) 5000 I0 27 l .033 37 6000 1.5 2.3.l 46

TABLE IV Stress Rupture Data For ALUMINIZED Alloy Steels at l300F in AirStress Level (psi) Rupture Life (hrs) The data in Tables Ill and Windicate that the aluminum coated low alloy titanium-molybdenum mildsteel panels of the present invention have the rupture life thereof atl.300F in air improved by a factor of IO. as compared with aluminumcoated low titanium alloy mild steel panels. The creep data also showthe extraordinary beneficial effect of adding a small amount ofmolybdenum to a low titanium mild steel. as evidenced fused into thesurface of steel sheet with only discrete particle of metallic oxideformed below the surface of said steel as a discontinuous subsurfacestratum; whereby said aluminum coated low alloy mild steel articleexhibits increased oxidation resistance, high creep resistance andincreased rupture strength on heating the said elevated temperature inan oxidizing atmosphere.

2. An article as in claim 1. wherein an excess of titanium remainsuncombined in said steel in an amount between about 0.1 and 0.3 percentby weight of said steel.

3. An article as in claim 1, wherein said low-alloy low-carbon steelcontains a maximum of about 0. 10 wt. percent carbon. a maximum of about0.01 wt. percent nitrogen. a maximum of about 0.70 wt. percentmanganese. and a maximum of about 0.09 wt. percent aluminum with thebalance being essentially iron with the normal amount of incidentalimpurities contained in a mild steel.

4. An article as in claim 1, wherein said coating of aluminum issubstantially pure aluminum.

5. An article as in claim 1, wherein said coating of aluminum containssilicon up to about 11 percent by weight silicon.

6. A method of providing an aluminum coated low alloy mild steel sheetwhich has good formability at room temperature and high creep resistanceand rupture strength at an elevated temperature which comprises.applying a uniform thin coating of metallic aluminum directly to asurface of a steel sheet which is free surface oxides and non-metallicimpurities and which is formed of a low-alloy aluminum killed low carbonsteel having distributed uniformly throughout said steel as an essentialcombination of alloy element titanium in an amount up to 1% whichcombines with all the carbon and nitrogen in the said steel andmolybdenum in an amount between about 0.10 and 0.70 percent by weight.

7. A method as in claim 6. wherein an excess of titanium remainsuncombined in said steel in an amount between about 0.1 percent and 0.3percent by weight of said steel base.

8. A method as in claim 6. wherein said steel base contains a maximum ofabout 0.10 wt. percent carbon. a maximum of about 0.01 wt. percentnitrogen, a maximum of about 0.70 wt. percent manganese. and a maximumof about 0.09 wt. percent aluminum with the balance being essentiallyiron with the normal amounts of impurities contained in a low carbonsteel.

9. A method as in claim 6, wherein saidaluminum is applied to said steelarticle by immersing said steel article (material) in a bath of moltenaluminum.

10. A method as in claim 9, wherein said molten aluminum bath issubstantially pure aluminum.

11. A method of providing an aluminum coated low alloy steel sheet whichhas good formability at room temperature and high creep resistance andrupture strength at an elevated temperature which comprises, adjustingthe composition of an aluminum killed lowcarbon steel which contains amaximum of about 0.10 wt. carbon. a maximum of about 0.01 wt. nitrogen.a maximum of about 0.70 wt. manganese. and a maximum of about 0.09 wt.aluminum with the balance being essentially iron with the normal amountof incidental impurities contained in a low-carbon aluminum killed steelby adding to said steel while said steel is in a molten condition as anessential combination of alloy elements titanium in an amount up to 1%by wt. which combines with all the carbon and nitrogen in the steel andleave an excess of uncombined titanium distributed throughout the steeland molybdenum in an amount between about 0.10 percent and 0.70 percentby weight to form a low-alloy aluminum killed low carbon steel andapplying directly to a surface of a sheet of said lowalloy aluminumkilled low-carbon steel while said surface is free of oxides andnon-metallic impurities a thin uniform coating of molten metallicaluminum. and cooling said coating to provide an aluminum coatedlow-alloy steel sheet having good formability at room temperature andhaving high creep resistance and rupture strength at elevatedtemperatures up to 1500F.

I 1B i i

1. AN ALUMINUM COATED LOW ALLOY MILD STEEL ARTICLE ADAPTED FOR SERIVICEAT AN ELEVATED TEMPERATURE COMPRISING, A LOW-ALLOY ALUMINUM KILLED LOWCARBON STEEL SHEET WHICH CONTAINS UNIFORMLY DISTRIBUTED THROUGHOUT AS ANESSENTIAL COMBINATION OF ALLOY ELEMENTS TITANIUM IN AN AMOUNT UP TO AMIXIMUM OF 1% BY WT. WHICH COMBINES WITH ALL THE CARBON AND NITROGEN ANDLEAVING UNCOMBINED TITANIUM THROUGHOUT THE STEEL AND MOLYBDENUM IN ANAMOUNT BETWEEN ABOUT 0.10 PERCENT AND 0.70 PERCENT BY WT. OF SAID STEEL,A UNIFORM THIN COATING OF METALLIC ALUMINUM DIRECTLY ON A SURFACE OFSAID STEEL SHEET WHICH IS FREE OF SURFACE OXIDES AND NON-METALLICIMPURITIES, SAID ARTICLE IN THE AS COATED CONDITION EXHIBITING GOODFORMABILITY AT ROOM TEMPERATURE AND HIGH CREEP RESISTANCE AND RUPTURESTRENGTH AT AN ELEVATED TEMPERATURE OF 1300*F-1500*F, AND SAID ARTICLEAFTER HEATING IN AN OXIDIZING ATMOSPHERE AT A TEMPERATURE OF 1500*F ANDSAID ARTICLE HAVING ALUMINUM UNIFORMLY DIFFUSED INTO THE SURFACE OFSTEEL SHEET WITH ONLY DISCRETE PARTICLE OF METALLIC OXIDE FORMED BELOWTHE SURFACE OF SAID STEEL AS A DISCONTINUOUS SUBSURFACE STRATUM, WHEREBYSAID ALUMINUM COATED LOW ALLOY MILD STEEL ARTICLE EXHIBITS INCREASEDOXIDATION RESISTANCE, HIGH CREEP RESISTANCE AND INCREASED RUPTURESTRENGTH ON HEATING THE SAID ELEVATED TEMPERATURE IN AN OXIDIZINGATMOSPHERE.
 2. An article as in claim 1, wherein an excess of titaniumremains uncombined in said steel in an amount between about 0.1 and 0.3percent by weight of said steel.
 3. An article as in claim 1, whereinsaid low-alloy low-carbon steel contains a maximum of about 0.10 wt.percent carbon, a maximum of about 0.01 wt. percent nitrogen, a maximumof about 0.70 wt. percent manganese, and a maximum of about 0.09 wt.percent aluminum with the balance being essentially iron with the normalamount of incidental impurities contained in a mild steel.
 4. An articleas in claim 1, wherein said coating of aluminum is substantially purealuminum.
 5. An article as in claim 1, wherein said coating of aluminumcontaiNs silicon up to about 11 percent by weight silicon.
 6. A methodof providing an aluminum coated low alloy mild steel sheet which hasgood formability at room temperature and high creep resistance andrupture strength at an elevated temperature which comprises, applying auniform thin coating of metallic aluminum directly to a surface of asteel sheet which is free surface oxides and non-metallic impurities andwhich is formed of a low-alloy aluminum killed low carbon steel havingdistributed uniformly throughout said steel as an essential combinationof alloy element titanium in an amount up to 1% which combines with allthe carbon and nitrogen in the said steel and molybdenum in an amountbetween about 0.10 and 0.70 percent by weight.
 7. A method as in claim6, wherein an excess of titanium remains uncombined in said steel in anamount between about 0.1 percent and 0.3 percent by weight of said steelbase.
 8. A method as in claim 6, wherein said steel base contains amaximum of about 0.10 wt. percent carbon, a maximum of about 0.01 wt.percent nitrogen, a maximum of about 0.70 wt. percent manganese, and amaximum of about 0.09 wt. percent aluminum with the balance beingessentially iron with the normal amounts of impurities contained in alow carbon steel.
 9. A method as in claim 6, wherein said aluminum isapplied to said steel article by immersing said steel article (material)in a bath of molten aluminum.
 10. A method as in claim 9, wherein saidmolten aluminum bath is substantially pure aluminum.
 11. A method ofproviding an aluminum coated low alloy steel sheet which has goodformability at room temperature and high creep resistance and rupturestrength at an elevated temperature which comprises, adjusting thecomposition of an aluminum killed low-carbon steel which contains amaximum of about 0.10 wt. % carbon, a maximum of about 0.01 wt. %nitrogen, a maximum of about 0.70 wt. % manganese, and a maximum ofabout 0.09 wt. % aluminum with the balance being essentially iron withthe normal amount of incidental impurities contained in a low-carbonaluminum killed steel by adding to said steel while said steel is in amolten condition as an essential combination of alloy elements titaniumin an amount up to 1% by wt. which combines with all the carbon andnitrogen in the steel and leave an excess of uncombined titaniumdistributed throughout the steel and molybdenum in an amount betweenabout 0.10 percent and 0.70 percent by weight to form a low-alloyaluminum killed low carbon steel and applying directly to a surface of asheet of said low-alloy aluminum killed low-carbon steel while saidsurface is free of oxides and non-metallic impurities a thin uniformcoating of molten metallic aluminum, and cooling said coating to providean aluminum coated low-alloy steel sheet having good formability at roomtemperature and having high creep resistance and rupture strength atelevated temperatures up to 1500*F.